1. Field of the Invention
The present invention relates to semiconductor devices in which a ladder resistor used in an analog-to-digital converter circuit (hereinafter, referred to as an A/D converter circuit) that converts analog data into digital data or a digital-to-analog converter circuit (hereinafter, referred to as a D/A converter circuit) that converts digital data into analog data is formed on a semiconductor substrate.
2. Description of the Related Art
In A/D converter circuits or D/A converter circuits, a ladder resistor including a plurality of resistors arranged in series with each other in order to generate a plurality of standard voltages or a plurality of reference voltages has been used. Due to recent development in integration technologies, semiconductor devices in which an A/D converter or a D/A converter that includes a ladder resistor is formed by a semiconductor process have been produced.
Such a known ladder resistor produced by the semiconductor process has a configuration in which, as shown in FIG. 4A (top view) and FIG. 4B (cross-sectional view), a plurality of metal wires, a plurality of contacts, and a plurality of resistor elements are connected in series along a current path. That is, such a ladder resistor is configured such that, as shown in FIG. 5, a plurality of unit resistors each constituted by metal wires, contacts, and a resistor element (=2×Rm+2×Rc+Rp, where “Rm” represents a metal wire resistor, “Rc” represents a contact resistor, and “Rp” represents a poly resistor) are connected in series with each other. In addition, a plurality of standard voltages or a plurality of reference voltages is extracted from between the unit resistors. When a plurality of standard voltages or a plurality of reference voltages is generated using a ladder resistor having the above-mentioned configuration, the accuracy of acquisition of the voltage division ratio is determined in accordance with variations in metal wires, contacts, and resistor elements.
Thus, when the above-mentioned ladder resistor is used, the voltage division ratio is acquired less accurately, due to the variations in the metal wires, the contacts, and the resistor elements.
A ladder resistor configured to increase the accuracy of acquisition of the voltage division ratio is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 5-284031. In the ladder resistor, as shown in FIG. 6 A (top view) and FIG. 6B (cross-sectional view), in order to prevent a reduction in the accuracy of acquisition of the voltage division ratio due to the variations in metal wires and contacts, the metal wires and the contacts are provided in portions other than an area that substantially serves as a current path.
In recent years, an improvement in the resolution of A/D converter circuits or D/A converter circuits has been desired. In accordance with this, a further improvement in the accuracy of acquisition of the voltage division ratio has also been desired.
The ladder resistor described in Japanese Unexamined Patent Application Publication No. 5-284031 is also capable of preventing a reduction in the accuracy of acquisition of the voltage division ratio due to the variations in metal wires and contacts. However, in order to further improve the accuracy of acquisition of the voltage division ratio, it is necessary to improve the accuracy of acquisition of the resistance of a resistor element portion.
As a method for improving the accuracy of acquisition of the resistance of a resistor element portion, a method for increasing an area of a resistor element is considered. However, a resistor element having a large area has a weak strength against bending stress.